Final answer:
An increase in heart rate during inspiration is controlled by the autonomic nervous system, specifically the sympathetic nervous system stimulation of the SA node. The parasympathetic nervous system usually predominates, slowing the heart rate via the vagus nerve. During inspiration, reduced parasympathetic activity allows the heart rate to increase slightly.
Step-by-step explanation:
The slight increase in heart rate due to inspiration and the activation of the SA node is controlled by the autonomic nervous system (ANS). Specifically, it is the sympathetic nervous system component of the ANS that stimulates the SA node, leading to a faster heart rate during inspiration, often referred to as respiratory sinus arrhythmia.
Parasympathetic stimulation, originating from the cardioinhibitory region and mediated by the vagus nerve (cranial nerve X), releases acetylcholine (ACh) at the neuromuscular junction. This neurotransmitter acts to open potassium channels, slowing the heart rate by extending repolarization and increasing the time before the next spontaneous depolarization. Normally, this parasympathetic influence is dominant, but during inspiration, sympathetic stimulation slightly overrides it to accelerate the heart rate.
The SA node, often called the pacemaker of the heart, is prone to increase the heart rate when parasympathetic stimulation is decreased. Thus, during inspiration, a decrease in vagal activity allows the heart to beat faster, which is a necessary physiological adjustment to accommodate the changing pressures in the thorax that affect venous return to the heart.